Optical cache memory

ABSTRACT

An optical memory having an input port for receiving an input optical signal to be stored in the optical memory is disclosed. A portion of the optical signal is coupled to a storage loop for storing optical signals by a coupler that transfers a portion of the input optical signal to the storage loop. An optical signal stored in the storage loop is output by coupling a portion of that optical signal to a first external optical waveguide. The storage loop includes a semiconductor optical amplifier for amplifying the signals stored in the storage loop to compensate for losses incurred by those signals in traversing the storage loop. A plurality of such optical memories can be combined to form a larger memory that includes a reconditioning circuit that resets the amplitude of the optical signals to a value that depends on the amplitude of the optical signals.

FIELD OF THE INVENTION

The present invention relates to storage systems for optical data.

BACKGROUND OF THE INVENTION

To simplify the following discussion, the present invention will beexplained utilizing optically-based communication systems. However, thepresent invention can also be utilized in a wide range of otherapplications as will be discussed in more detail below.

The ever-increasing bandwidth requirements of communication systems haveresulted in data being transmitted over optical fibers. Bothconventional telecommunications and data networks such as the Internetutilize optical fibers for long distance transmission. Data is sent bybreaking the transmission into data packets that are routed between thesender and receiver with the aid of switches that route packets betweenfibers and/or channels within a single fiber.

While the transmission of the data is performed optically, the switchingof the data must often be performed by converting the optical signals toelectrical signals. The electrically-based data packets are thenre-arranged using an electrically-based cross-connect switch. There-arranged electrically-based packets are then converted back tooptical signals. The electrically-based switches typically switchpackets between a plurality of input channels and a plurality of outputchannels. Data from each input channel is stored in a cache memory. Thedata packets for each output channel are then assembled by reading thedata stored in the cache memory.

While optical communication channels provide data rates in excess of 10Gbits/sec at relatively modest costs, electrically based circuitry thatcan operate at these high data rates are either non-existent or veryexpensive. For example, the electrically based packet switches discussedabove must utilize a highly parallel architecture to make up for therelatively slow operating speed of the memories and data routingcircuitry to obtain the needed throughput. Such circuitry is complex andcostly.

SUMMARY OF THE INVENTION

The present invention includes an optical memory having an input portfor receiving an input optical signal to be stored in the opticalmemory. A portion of the optical signal is coupled to a storage loop forstoring optical signals by a coupler that transfers a portion of theinput optical signal to the storage loop. An optical signal stored inthe storage loop is output by coupling a portion of that optical signalto a first external optical waveguide. The storage loop includes asemiconductor optical amplifier for amplifying the signals stored in thestorage loop to compensate for losses incurred by those signals intraversing the storage loop. In one embodiment, the input port includesan input optical switch for connecting and disconnecting the storageloop from a second external optical waveguide. In another embodiment,the storage loop includes an optical switch for selectively blocking oneof the optical signals stored in the storage loop from propagating inthe storage loop. The optical switches are preferably operated by acontroller that monitors the storage loop and the input and outputports. A plurality of such optical memories can be combined to form alarger memory that includes a reconditioning circuit that resets theamplitude of the optical signals to a value that depends on theamplitude of the optical signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of one embodiment of an optical cachememory according to the present invention.

FIG. 2 illustrates the various signals during the introduction of anoptical data packet 31 on optical fiber 11 into storage loop 13.

FIG. 3 illustrates the various signals during the extraction of datapacket 31 from storage loop 13.

FIG. 4 is a schematic drawing of another embodiment of an optical cachememory according to the present invention.

FIG. 5 is a schematic drawing of another embodiment of an optical cachememory according to the present invention.

FIG. 6 is a schematic drawing of a plurality of optical cache memoriescombined to form a dual ported buffer memory.

FIG. 7 illustrates an exemplary embodiment of a buffer memory 300 havinga reconditioning circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present invention avoids the costly circuitry discussed above byproviding an optically based cache for storing optical data packets. Themanner in which the present invention stores an optical signal can bemore easily understood with reference to FIG. 1, which is a schematicdrawing of an optical cache memory 10 according to one embodiment of thepresent invention. Optical cache memory 10 stores and retrieves opticalpackets in the form of a modulated optical signal that specifies aplurality of bits of data. Any appropriate form of optical modulationcan be utilized including amplitude modulation, phase modulation, andpolarization modulation. An optical packet that is to be stored inoptical cache memory 10 is input to optical cache memory 10 on opticalfiber 22 through optical switch 12. A portion of the light traversingoptical fiber 11 is diverted into storage loop 13 by coupler 14. Eachtime the optical packet traverses storage loop 13, a portion of thecirculating signal is coupled back to optical fiber 11 by coupler 14 andthe remainder of the signal re-circulates in storage loop 13. Theportion of the optical signal that is coupled back into optical fiber 11can be output via optical switch 16 onto output fiber 17. When opticalswitch 16 is closed, it absorbs the portion of the stored optical signalthat enters the switch. A controller 20 controls the various opticalswitches.

Optical switches are known to the art, and hence, will not be discussedin detail here. For the purposes of the present discussion, it issufficient to note that an optical switch can be constructed in a mannersimilar to an edge-emitting laser. Both the laser and modulator areconstructed by depositing a number of layers on a suitable substrate.The bottom layers typically include an n-type contact layer and acladding layer. An active region is grown on top of the cladding layer.The active layer may include one or more strained quantum well layersseparated by barrier layers. A confinement layer is typically depositedon each side of the active region. A p-type cladding layer and a p-typecontact layer are deposited on the active region. A light modulator orswitch is based on the observation that the quantum well structures willabsorb light below a cutoff wavelength whose value depends on thepotential across the quantum well layers. The position of the cutoffwavelength is determined by the composition of the quantum well layers.To provide the desired switching function, this cutoff wavelength mustmove from a value below the wavelength of the light signal beingswitched to a value above the wavelength of the light signal beingswitched when the potential across the modulator layers is switched.

As noted above, each time the stored packet traverses storage loop 13 aportion of the optical signal leaves storage loop 13, and hence, islost. Hence, in the absence of some form of amplifier, the signal instorage loop 13 would quickly fade. The required amplification isprovided by semiconductor optical amplifier 15 which has a gain that isset to just make up the losses resulting from absorption in the opticalfiber and the amplitude that is lost due to coupler 14. Semiconductoroptical amplifiers are also known to the art, and hence, will not bediscussed in detail here. For the purposes of the present discussion, itis sufficient to note that a semiconductor optical amplifier can beconstructed from a structure similar to that of an edge-emitting laser.However, the active region of a semiconductor optical amplifier may beconstructed from a bulk material rather than the strained quantum wellstructure utilized in such lasers. It should also be noted that the gainof a semiconductor optical amplifier depends on the intensity of thelight being amplified. As the intensity increases, the gain decreases.This gain effect provides a stabilization mechanism to guarantee thatthe light signal will not grow rapidly if the gain is set slightlyhigher than needed to make up for the intensity losses incurred by alight signal traversing the storage ring 13 and coupler 14.

Since the optical switches have the same basic structure as an opticalamplifier when the switch is biased to transmit light, these switchescan also provide gain as well as a switching function. Hence, the outputswitch may include sufficient gain to account for any losses in thecoupler suffered by the light signal as it exits the storage ring. Forexample, consider a case in which coupler 14 splits the light signalevenly between storage ring 13 and output port 16. Also assume that thelosses in the storage ring are small. In this case, both semiconductoroptical amplifier 15 and output switch 16 would be designed to have again of 2.

The number of bits that can be stored in storage loop 13 at any timedepends on the length of the optical fiber used to construct storageloop 13 and the modulation frequency, i.e., the bit rate, used to createthe data packet. For example, at a data rate of 10 Gbits/sec, a 50-ftstorage loop can accommodate approximately 750 bits. A storage loop ofthis size can be constructed by winding a conventional optical fiber ona cylindrical form or by utilizing lithographic techniques to form awound fiber loop on a cylindrical substrate. For example, co-pendingpatent application Ser. No. 10/219,968 filed Aug. 14, 2002 discloses amethod for fabricating a wave guide on the surface of a cylindricalsubstrate in which 10 meters of wave guide can be accommodated on acylindrical substrate that is 1 cm in diameter and 5 cm in length.

In one preferred embodiment of the invention, the length of storage loop13 is chosen such that storage loop 13 will hold a plurality of opticalbits. In principle, an optical cache memory according to the presentinvention could be constructed with a storage loop that held only onebit of optical data. However, such a storage loop would be of limitedvalue in constructing cache memories for use in communication systems inwhich the data is divided into packets of a larger size. As will beexplained in more detail below, an optical cache memory for use inconstructing a packet switch preferably holds at least one data packet.In general, a cache memory having a storage loop that holds more thanone data packet will cost less than a cache memory with a storage loopthat holds only one packet, since the extra storage capacity requiresonly that a longer storage loop be employed. Optical fiber is relativelyinexpensive compared to high-speed optical switches and semiconductoroptical amplifiers, and hence, the cost per bit stored decreases withthe size of storage loop 13.

The maximum size of storage loop 13 is determined by the maximum delaythat can be tolerated in retrieving a data packet. Any given data packetis only available for output when that data packet traverses opticalcoupler 14; hence, the maximum delay that can be incurred in reading adata packet is the transmission time required for an optical signal totraverse the full length of optical storage loop 13. Accordingly, thereis a tradeoff between the maximum data latency and the size of storageloop 13.

An optical cache memory according to the present invention can be usedto store a single data packet or multiple data packets. Refer now toFIG. 2, which illustrates the various signals during the introduction ofan optical data packet 31 on optical fiber 11 into storage loop 13 whilestorage loop 13 is currently storing a number of other data packetsshown at 32-34. If one were to view the light intensity in storage loop13 at coupler 14 as a function of time, the graph shown at 40 would beobtained. The storage capacity of storage loop 13 can be viewed as beingdivided into a plurality of time slots 41-44. In general, the time slotsmust include a buffer region at the beginning and end of the time slotto provide time for the various optical switches to open and closeduring storage and retrieval operations.

The introduction of data packet 13 must be timed such that there is afree slot at coupler 14 when the data packet arrives at input switch 12.If this is the first data packet to be placed in the storage loop, therewill always be such a slot. In general, controller 20 keeps track of thepositions of each of the stored data packets in storage loop 13 andsignals the source of the new data packet when it is safe to transmitthe new data packet. If the arrival of the new data packet must bedelayed, a dedicated cache memory according to the present invention canalso be used to provide that data delay. Controller 20 then opens switch12 to allow the data packet to enter storage loop 13.

An optical data packet is extracted from storage loop 13 in an analogousmanner to that used to write the optical data packet into storage loop13. Refer now to FIG. 3, which illustrates the various signals duringthe extraction of data packet 31 from storage loop 13. The data packetis extracted as it passes through coupler 14 by opening output opticalswitch 16.

The data packet is available for extraction each time it passes coupler14. If the time needed to traverse the storage loop is T, then a datapacket introduced at time t=0 will be available at times nT, where n isa positive integer. Hence, as noted above, a cache memory according tothe present invention can also be used to provide a digital delay linefor altering the relative timing of two optical data packets by storingone of the data packets in an optical cache according to the presentinvention.

In general, a packet that is read out of cache memory 10 needs to beremoved from storage loop 13. As noted above, only a portion of theoptical energy stored in storage loop 13 leaves via switch 16. Theremainder re-circulates in storage loop 13 and is amplified to make upfor the lost signal energy. Hence, a separate method is needed to removea packet from storage loop 13 once it is no longer needed. In thepreferred embodiment of the present invention, an optical switch 21similar to optical switch 16 is included in storage loop 13. Opticalswitch 21 is under the control of controller 20; however, to simplifythe drawing, the control connections between optical switch 21 andcontroller 20 have been omitted from the drawing. Optical switch 21 isnormally transparent. When a packet is to be extinguished, opticalswitch 21 is set to an opaque state for the time interval needed toabsorb the unwanted packet.

It should be noted that a cache according to the present invention mightalso be used to implement a broadcast function. As noted above, a packetthat is read from storage loop 13 is not eliminated from storage loop 13by the act of reading the packet. Hence, a packet can be read out at aplurality of different times to different final locations to implement abroadcast switch.

The embodiments of the present invention discussed above have a singleinput and a single output port. However, embodiments having multipleinput and output ports may also be constructed. Multi-port cachememories are particularly useful building blocks for packet switchessince the multiple output ports can be connected to different outputfibers, thereby allowing packets input on one or more input fibers to besorted to a different set of output fibers.

Refer now to FIG. 4, which is a schematic drawing of an optical cachememory 100 according to another embodiment of the present invention.Optical cache memory 100 has two input ports and two output ports. Theinput ports include fibers 101 and 113, respectively, which receive thepackets to be stored in a storage ring 123. The output ports includefibers 111 and 103, respectively, which receive the outbound packets.Optical cache memory 100 utilizes couplers 121 and 122 to transferoptical signals to and from storage ring 123. The loses incurred atcouplers 121 and 122 are made up by semiconductor optical amplifiers 125and 124, respectively. Signals are gated into storage ring 123 by inputoptical switches 102 and 114. Similarly, optical signals that leave thestorage ring via the couplers are gated to the output fibers by outputoptical switches 104 and 112.

As noted above, an optical cache memory according to the presentinvention preferably includes an optical switch in the storage ring forblocking the propagation of packets that are to be dropped from thering. Optical cache memory 100 preferably utilizes two such opticalswitches 127 and 128. However, only one optical switch for blockingunwanted packets is needed. The use of one such blocking optical switchper output port has the advantage of allowing a packet that has beenoutput to be erased before it reaches a second coupler in the loopthereby freeing the storage slot for a new packet to be introduced atthe second coupler.

The use of multiple output ports also reduces the data latency for aread operation. As noted above, the data latency for a read operation isthe time needed for the desired data packet to traverse the portion ofthe storage loop between the packets current location and the outputport through which it is to be read. If the optical cache memory onlyhas one data output port, the maximum data latency is the time totraverse the entire storage loop. In contrast, if N output ports areutilized, this time can be reduced by a factor M, assuming that thepacket can be utilized at any output port.

It should be noted that timing considerations are important in opticalsystems because of the lack of other forms of optical cache memories.Refer now to FIG. 5, which is a schematic drawing of another embodimentof an optical cache memory according to the present invention. Tosimplify the following discussion, those elements of optical cachememory 250 that serve the same function as the corresponding elements inoptical cache memory 10 shown in FIG. 1 have been given the same numericdesignations. Optical cache memory 250 stores data packets 215 that arepresent on fiber 220 in storage loop 13. For the purposes of thisdiscussion, it will be assumed that controller 202 is programmed suchthat controller 202 will cause a packet on fiber 220 to be stored ifthere is a free slot in storage loop 13 that will be available atcoupler 14 when packet 215 arrives at coupler 14. In this case, opticalcache memory 250 will prevent packet 215 from propagating further downfiber 220 by setting optical switch 216 to block packet 215. If no suchslot is available, optical cache memory 250 allows packet 215 tocontinue down fiber 220 where it will presumably be stored in anothercache memory that is connected to fiber 220. To provide sufficient timeto set optical switches 12 and 216, a delay line 217 is introducedbetween the point on fiber 220 that is sampled by controller 202 andswitches 12 and 216. In this embodiment of the invention, the input todelay line 217 may be viewed as the input port to optical cache memory250 and the output of optical switch 216 will be referred to as aby-pass port.

Optical cache memory 250 outputs a data packet in an analogous manner inresponse to a WRITE control signal indicating which packet stored instorage loop 13 is to be output. The indicated packet will be outputonto fiber 221 in the next available slot on fiber 221 that coincideswith the packet being available at output switch 16. Controller 202preferably monitors fiber 221 to determine the presence of an availableslot for the packet in question. A delay line 218 for providing a onepacket delay is preferably utilized between the sampling point on fiber221 and the output of optical switch 16. This delay allows controller202 to determine that a slot of the required length is available onfiber 221.

The read and write operations described above can be performed inresponse to a specific read or write command being provided tocontroller 202 or in response to some predetermined algorithm. Forexample, controller 202 can be programmed to store any packet that isavailable on fiber 220 that matches a blank slot in storage loop 13.Alternatively, controller 202 can be programmed to only accept such apacket if it is specified in a read command. In one embodiment of thepresent invention, controller 202 includes circuitry for reading theheader bits on the packets and utilizing this information to determineif the packet is to be stored and later read out onto fiber 221.

Similarly, packets stored in storage loop 13 can be read out in responseto a specific command or automatically. For example, if optical cachememory 250 is part of a packet switch, controller 202 can be programmedto accept all properly timed packets having a specified bit set in theheader of the packet. The header can be read through a fiber 212. Apacket would then be read out onto fiber 221 as soon as space for thepacket is available. In this scheme, the optical cache memory performs asorting function for switching packets having specified headers fromfiber 220 to fiber 221.

A plurality of optical cache memories can be combined to form a dualported buffer memory. Refer now to FIG. 6 which is a schematic drawingof a buffer memory 200 constructed in this manner. Buffer memory 200 isconstructed from a plurality of optical cache memories similar tooptical cache memory 250 described above. Exemplary optical cachememories are shown at 201-202. The optical cache memories accept opticalpackets 205 from an input fiber 203 and output the stored packets onoutput fiber 204. As each packet approaches an optical cache memory, theoptical cache memory determines if there is a slot for that packet inthe storage loop in that optical cache memory. If there is a free slot,that optical cache memory diverts the packet into its storage loop. Ifnot, the packet proceeds to the next optical cache memory on the inputoptical fiber and the process is repeated until the optical packet isfinally stored in one of the optical cache memories.

If sufficient optical cache memories are provided in the packet switch,each packet will eventually be stored in a storage loop on one of theoptical cache memories. If there are not sufficient optical cachememories, some of the packets will reach the end of the input fiberwithout finding a storage loop. The number of such packets will beconsiderably smaller than the number of packets arriving at the input tobuffer memory 200. Accordingly, an electrically based overflow processor206 can be provided to temporarily store these remaining packets byconverting the optical packets to electrical signals and storing theelectrical signals.

Stored packets are output on optical fiber 204 as space is available onthat optical fiber or in response to a specific output command sent tothe controllers in the optical cache memories. For example, buffermemory 200 can be operated as a FIFO buffer by outputting the packets inthe order in which the packets were stored. In this case, the variouscontrollers keep track of the time at which a packet was placed in thestorage loop associated with that controller. Each controller alsotransmits that information to the other controllers such that thecontroller with the oldest packet will be given priority to output thatpacket on output fiber 204 in the next slot that becomes available aftera readout command is received by buffer memory 200.

The control functions associated with buffer memory 200 can bedistributed between the various controllers in the optical cachememories or may be centralized in a single controller. In addition,systems in which both a centralized controller and local controllersshare the control functions can be constructed. To simplify the drawing,only the local controllers are shown in the drawing.

The above-described embodiments of the present invention assume that theoptical signals stored in the storage loops remain uncorrupted by noiseduring the time the signals are stored in the storage loops. However, ifthe signals remain in the storage loops for a sufficiently long periodof time, sufficient noise will accumulate to corrupt the signals. Forexample, the semiconductor optical amplifiers will introduce some noiseinto the signals on each passage of a signal through the amplifier.Accordingly, some form of signal reconditioning may be advantageous insome embodiments of the present invention.

To recondition a signal, the optical signal is preferably sent through aprocessor that compares the signal to a predetermined threshold and setsthe signal to zero if the signal is less than that threshold and to anamplitude corresponding to a logical one if the signal is greater thanthat threshold. Such circuits are well known in the digital electronicarts; however, an economical optical equivalent circuit is not known.Accordingly, the present invention preferably converts an optical signalto be reconditioned to an electrical signal and then performs thereconditioning on the electrical signal. The reconditioned electricalsignal is then converted back into an optical signal. Suchreconditioning circuitry is known to the art, and hence, will not bediscussed in detail here.

As noted above, electrically based circuitry that must operate at thefrequencies used to modulate optical signals is quite expensive. Hence,the present invention preferably utilizes an arrangement in which anumber of optical cache memories according to the present inventionshare a single reconditioning circuit, thereby reducing the effectivecost per optical cache memory. Refer now to FIG. 7, which illustrates anexemplary embodiment of a buffer memory 300 having a reconditioningcircuit. Buffer memory 300 is constructed from a plurality of opticalcache memories that operate in a manner similar to that described above.Exemplary memories are shown at 301-303. Optical cache memories 301-303are preferably the same as optical cache memory 250 shown in FIG. 5. Tosimplify the drawing, only the components involved in the reconditioningoperation are shown. In addition, the switches that correspond toswitches 216-218 shown in FIG. 5 have also been omitted from the drawingtogether with the controllers.

The optical cache memories store data packets presented on an inputoptical fiber 320 and output data packets onto an output optical fiber321. When the data in one of the optical cache memories requiresreconditioning, that optical cache memory outputs its data toreconditioning circuit 310. The reconditioned data is then presented asinput data on optical fiber 320. A switch 322 prevents the data that isbeing reconditioned from exiting from the buffer memory during thereconditioning operation.

The above-described embodiments of the present invention have utilizedoptical fibers as the transmission medium for the optical signals.However, other suitable forms of waveguide can be employed withoutdeparting from the teachings of the present invention.

The above-described embodiments of the present invention have utilizedspecific types of optical switches and amplifiers. However, it is to beunderstood that other types of optical switches and amplifiers may beutilized.

Various modifications to the present invention will become apparent tothose skilled in the art from the foregoing description and accompanyingdrawings. Accordingly, the present invention is to be limited solely bythe scope of the following claims.

1. An optical memory comprising: an input port for receiving an inputoptical signal; a storage loop for storing a plurality of opticalsignals; a coupler for transferring a first portion of said inputoptical signal to said storage loop; an output switch for coupling asecond portion of one of said optical signals stored in said storageloop to a first external optical waveguide while leaving a third portionof that optical signal in said storage loop; and a semiconductor opticalamplifier for amplifying said optical signals stored in said storageloop.
 2. The optical memory of claim 1 wherein said input port comprisesan input optical switch separate from said coupler for connecting anddisconnecting said storage loop from a second external opticalwaveguide.
 3. The optical memory of claim 1 wherein said storage loopfurther comprises an optical switch for selectively blocking one of saidoptical signals stored in said storage loop from propagating in saidstorage loop.
 4. The optical memory of claim 1 further comprising acontroller for operating said output optical switch.
 5. An opticalmemory comprising: an input port for receiving an input optical signal;a storage loop for storing optical signals; a coupler for transferring afirst portion of said input optical signal to said storage loop; anoutput switch for coupling a second portion of one of said opticalsignals stored in said storage loop to a first external opticalwaveguide; a a semiconductor optical amplifier for amplifying saidoptical signals stored in said storage loop; and a controller foroperating said output optical switch, wherein said controller furthercomprises a decoder for reading optical signals stored in said opticalstorage loop.
 6. An optical memory comprising: an input optical fiberfor receiving optical data packets to be stored in said optical memory;an output optical fiber for reading out optical data packets stored insaid optical memory; a plurality of cache optical memories, each cacheoptical memory comprising: an input port connected to said input opticalfiber for selectively receiving one of said optical data packets on saidinput optical fiber, a storage loop for storing optical data packets; acoupler for transferring a first portion of said one of said opticaldata packets to said storage loop; an output switch for coupling asecond portion of one of said optical data packets stored in saidstorage loop to said output optical fiber, while leaving a third portionof that optical packet in said storage loop; and a semiconductor opticalamplifier for amplifying said optical data packets stored in saidstorage loop.
 7. An optical memory comprising: an input optical fiberfor receiving optical data packets to be stored in said optical memory;an output optical fiber for reading out optical data packets stored insaid optical memory; a plurality of cache optical memories, each cacheoptical memory comprising: an input port connected to said input opticalfiber for selectively receiving one of said optical data packets on saidinput optical fiber; a storage loop for storing optical data packets; acoupler for transferring a first portion of said one of said opticaldata packets to said storage loop; an output switch for coupling asecond portion of one of said optical data packets stored in saidstorage loop to said output optical fiber; a semiconductor opticalamplifier for amplifying said optical data packets stored in saidstorage loop; and a controller for monitoring said input optical fiberfor an optical packet to be stored in said optical memory and forcausing one of said cache memories to store said optical packet if thatcache memory has space in said storage loop contained in said cachememory for that optical packet.
 8. The optical memory of claim 7 whereinsaid controller causes one of said cache optical memories to couple athird portion of one of said optical packets stored in that cacheoptical memory to said output optical fiber.
 9. The optical memory ofclaim 8 wherein said controller monitors said output optical fiber todetermine if said output optical fiber has space for said one of saidoptical packets.
 10. An optical memory comprising: an input opticalfiber for receiving optical data packets to be stored in said opticalmemory; an output optical fiber for reading out optical data packetsstored in said optical memory; a plurality of cache optical memories,each cache optical memory comprising: an input port connected to saidinput optical fiber for selectively receiving one of said optical datapackets on said input optical fiber; a storage loop for storing opticaldata packets; a coupler for transferring a first portion of said one ofsaid optical data packets to said storage loop; an output switch forcoupling a second portion of one of said optical data packets stored insaid storage loop to said output optical fiber; a semiconductor opticalamplifier for amplifying said optical data packets stored in saidstorage loop; and a reconditioning circuit for reacting optical packetsfrom said output fiber, said optical packets comprising a plurality ofoptical signals characterized by an optical signal amplitude, saidreconditioning circuit causing the optical signal amplitude of saidoptical signals to be set to a first value if said optical signalamplitude is greater than, or equal to, a threshold value and to asecond value if said optical signal amplitude is less than saidthreshold value.
 11. A method for storing an optical signal, said methodcomprising causing a first portion of said optical signal to be storedin an optical storage loop; amplifying said first portion of saidoptical signal stored in said optical storage loop to compensate forlosses in said optical storage loop; and causing a second portion ofsaid optical signal to be transferred to an output optical waveguide,while leaving a third portion of that optical packet in said storageloop.
 12. The method of claim 11 further comprising the elimination ofsaid first portion of said optical signal from said optical storage loopafter said second portion of said optical signal has been transferred tosaid output optical fiber.
 13. The method of claim 11 wherein said firstportion of said optical signal is amplified by a semiconductor opticalamplifier.
 14. The method of claim 11 wherein said output opticalwaveguide comprises an optical fiber.
 15. The method of claim 11 whereina controller determines if there is space in said optical storage loopprior to causing said first portion of said optical signal to be storedin said optical storage loop.